MTU of the underlying interfaces should be >=1528

[PaulSchaetzle]

See the official batman-adv documentation: https://www.open-mesh.org/projects/batman-adv/wiki/Fragmentation-technical. OpenWrt will also warn about this in the System Log. You simply need to set the "Override MTU" setting in the mesh interface to the desired MTU.

[benkay86]

You are right, I will need to update the tutorial.